Unless your school really, really encourages you to buy the newer versions dont waste your money. The last time I checked we still were made of the same tissues and had the same chemical pathways that the last edition said we had.
Most publishers rearrange the chapters and illustrations, and change the cover of the book, then packege the thing as a spanking new edition so students would pay for new books instead of buying old ones.
The one exception to this is if you know that exam questions will be coming from the new edition of a certain book, for example histology slides. Then you would be a dope not to invest.
Other than that stick to used books that your upperclassmen will be haking the first weeks of school. You pay less than half price.